This invention concerns a high- or medium- density extrudable and crosslinkable polyethylene composition. It also relates to a process for manufacturing extruded and crosslinked articles as well as the resulting aritcles.
It is known that crosslinking of high- or medium- density polyethylenes (which can be effected by a process involving reactions with radicals initiated by a free radical generator compound) improves some of their properties, particularly their mechanical properties and their resistance to heat.
It is known, on the other hand, that it is possible to shape by sintering high- or medium- density polyethylenes (crosslinkable by reaction involving radicals). However the usual sintering processes require the use of a specific apparatus and of high operating pressures. (On this subject, reference can be given, for example, to the French Pat. No. 2 383 006).
The shaping of polyethylenes by extrusion in a conventional apparatus is subject to certain difficulties.
On the one hand, as the crosslinking of high-density polyethylenes by free radical generators, effected after the shaping step (extrusion), requires the heating of the substance to be crosslinked up to the decomposition temperature of the radical generator compound, it is necessary, in order to avoid the deformation of the product, to make use of high-density polyethylenes of high molecular weight.
On the other hand, such polyethylenes cannot be extruded at a satisfactory rate, except at high temperatures which produce, as soon as the extrusion is performed, the decomposition of the usual free radical generators.
Various plasticizing agents for high-density polyethylenes are known (see for example "The technology of solvents and plasticizers" of Arthur K. DOOLITTLE--John WILEY & SONS, NEW YORK) which may be used in the presence of an initiator of radical reactions, such as a peroxide, at the temperatures required for the shaping of the material (e.g. extrusion). Plasticizing agents may be esters such, for example, as alkyl acrylates and methacrylates.
However, these plasticizing agents suffer from two major drawbacks.
On the one hand, in most cases, they react within the temperature range used for the extrusion, having the effect of initiating the cross-linking during the extrusion, which is thus disturbed.
On the other hand, they have an insufficient plasticizing effect when used at concentrations which preserve the satisfactory properties of the final crosslinked products.
Otherwise, the gel proportion of the products after crosslinking is often substantially reduced.